Our invention relates to high voltage terminations and, more particularly, to an improved terminal and bushing construction for certain high voltage power supplies.
Electrical power supply apparatus produce electrical voltages on the order of 10-15 kilo volts at power levels of 5-25 watts for supplying the high voltage electrical requirements of electronic equipment, such as photostatic copiers and electrostatic air cleaners. Typically, the electrical circuitry of the power supply apparatus, which raises the ordinary line voltages, such as 120 volts, 60 cycle AC, to the aforementioned voltage levels, is encased in a metal housing or "can" and any voids within the housing cavity are filled with a "potting" material, such as electrical grade pitch, which serves to exclude moisture and conduct heat from the encased circuitry to the housing walls. The high voltage output of the encased circuitry is coupled by an electrical lead to an elongated electrical terminal or electrode, as variously termed. The electrode physically extends through a housing wall supported and insulated by an electrical insulator, often referred to as a high voltage bushing or, simply, bushing. The high voltage for application to the electronic equipment is taken by connection to that electrode.
The electrical insulator and electrode construction of typical power supply units with which we are familiar is either a unitary structure in which the electrode is molded within the insulator or one which becomes a unitary structure once the electrode is encased in the insulating bushing in a subsequent assembly operation. With either type, rubber glands or rubber washers are incorporated to serve as a moisture barrier at the interface between the electrode-bushing assembly and the encased electrical circuitry. Additionally, the insulator bushing typically is of a coated porcelain, ceramic or glass material which, possessing a higher electric strength than air, prevents current leakage or discharge between the electrode and the metal enclosure which is usually at ground potential.
Occasionally, a glass bushing breaks or cracks and its ability to maintain a high degree of electrical insulation between the electrode and the housing is thereby substantially reduced. And in the case of a coated porcelain bushing, minute pinholes in the glazed coating may allow moisture absorption, thus rendering the insulator useless and requires replacement. Inasmuch as the bottom side of the insulator and electrode are located within and effectively bonds to hardened potting material in those units with which we are familiar, replacement of the high-voltage bushing and reconditioning of the power supply unit at the factory is difficult and repair of those units in the field is almost impossible from an economic standpoint. There exist also, structural arrangements which incorporate a metal-to-metal seal, such as solder, at the interface of the housing and bushing or of the electrode and the bushing or of both kinds which makes bushing replacement even less practical than in the preceding example.
The principal purpose of our invention is to provide a novel construction for such high voltage power supply units that allows quick, easy and effective field replacement of the high voltage bushing. A related purpose of the invention is to ensure that the bushing does not effectively bond to the potting material.